The present invention relates to an objective lens which is used for an optical disc drive capable of recording/reading data to/from two or more types of optical discs having different data densities and cover layer thicknesses.
There exist many types of optical discs according to various standards with different data densities and different thicknesses of the cover layer (protective layer). For instance, DVD (Digital Versatile Disc) has higher record density and a thinner cover layer than CD (Compact Disc). Therefore, when an optical disc loaded on an optical disc drive is replaced with another optical disc of a different type (standard), another effective beam diameter suitable for data density of the new disc has to be attained for reading/writing information while properly correcting spherical aberration which changes depending on the cover layer thickness.
For example, for realizing the information recording/readout on optical discs having high data density, a beam spot diameter has to be reduced by increasing an NA (numerical aperture) in comparison with an optical system specially employed for optical discs having low data density. Since the beam spot diameter gets smaller as the beam wavelength gets shorter, a laser light source emitting a laser beam having a wavelength of 635-665 nm has to be employed for optical systems for DVD, which is shorter than that (780-830 nm) of optical systems exclusively used for CD. Therefore, optical disc drives (optical information recording/readout devices) of recent years are provided with an optical system having a light source unit capable of emitting laser beams of different wavelengths.
In order for properly converging and focusing the laser beam on recording surfaces of optical discs having different cover layer thicknesses, an objective lens having a diffracting structure (which is partitioned into a plurality of annular zones by small level differences) on its one side is in practical use today for the optical systems for optical discs. Such an objective lens focuses the laser beam on the recording surface of the disc constantly with a proper NA even when optical discs of different standards are used, utilizing the characteristics of the diffracting structure causing different spherical aberrations depending on the wavelength of the incident beam.
The surface of such a conventional objective lens is divided into an inner area including the optical axis of the lens and an outer area outside the inner area. Typically, the inner area has diffracting structure capable of properly focusing the beam for the CD on the recording surface of the CD while also being capable of properly focusing the beam for the DVD on the recording surface of the DVD. Meanwhile, the outer area has diffracting structure capable of properly focusing the beam for the DVD on the recording surface of the DVD while being capable of preventing the proper focusing of the beam for the CD on the recording surface of the CD and thereby avoiding excessive convergence of the beam.
That is, part of the beam for the CD that passed through the outer area is diffused on the recording surface due to large spherical aberration caused by the diffracting structure, while only part of the beam for the CD that passed through the inner area is focused on the recording surface and forms a beam spot of a relatively large diameter. In the case of the beam for DVD, the NA becomes larger since part of the beam passing through the outer area also focuses on the recording surface, by which a small beam spot suitable for information recording/readout of DVD having high data density is formed on the recording surface.
Such a DVD/CD compatible objective lens and an optical information recording/readout device employing the DVD/CD compatible objective lens have been disclosed in Japanese Patent Provisional Publications No. P2001-216674A and P2001-249273A, for example.
In the case of writable optical discs such as CD-R and CD-RW, the intensity of the beam spot (on the recording surface) for writing is desired to be higher than that for reading. Ordinary objective lenses without the compatibility with optical discs of two or more standards are capable of forming a beam spot of high intensity on the recording surface using all the beam passing through the lens. On the other hand, in the conventional DVD/CD compatible objective lens, only the inner area contributes to the conversion and focusing of the beam for the information recording/readout of CD as mentioned above. Therefore, the use of the conventional DVD/CD compatible objective lens inevitably results in low light intensity of the beam spot compared to the case of the ordinary objective lenses distributed to only one type of optical disc.
In the conventional DVD/CD compatible objective lens, it is possible to increase the beam spot intensity by enlarging effective diameter of the area through which the beam for information recording/readout of CD passes, that is, by increasing the size (area) of the inner area. However, extending the inner area is accompanied by excessive convergence of the beam spot formed on the recording surface of CD, causing insufficiency of spot diameter for information readout of the CD, unexpected large aberration due to a tilt of the disc, etc., by which the permissible range avoiding read error narrows.